1. Field of the Invention
The present invention relates to an integrated ring sensor which senses a ringing signal and provides a signal indicating that a telephone is off-hook for both a short-loop condition and a power-cross condition.
2. Background of the Related Art
A terminal connected to a telephone line should periodically undergo a "power-cross" test. This test is performed in order to verify the ability of the terminal to detect the presence of a foreign potential, such as a high voltage of up to 1000 volts AC from a local power company or the like, which may become crossed with the telephone line in an environment outside of the telephone plant. FIG. 1 shows a telephone 10 connected to a telephone plant 12 through wires 14. A power line 16 poses a threat to the telephone plant 12 and any personnel operating telephone equipment that are connected to the wires 14 that are crossed with the power line 16.
The power-cross test typically involves forcing a 1 ampere current into tip and ring lines 18, 20 regardless of the state (i.e., talking, standby or ringing) of those lines, and checking whether the telephone plant 12 survives the test. Generally, in order to provide a 1 ampere current, a 1,000 volt, 60 hertz signal through a 1 kohm resistor is applied to the tip and ring lines 18, 20. Other combinations of voltages and resistance values, such as a 600 volt signal through a 600 ohm resistor, or a 50 volt signal through a 50 ohm resistor, could also be used to obtain the 1 ampere test current.
As mentioned above, the power-cross test is applied regardless of the state of the telephone plant 12. For example, a person could be talking on the telephone 10, or the telephone 10 could be in a standby mode, or the telephone 10 could be ringing and on-hook. Conventionally, during either the talking mode or the standby mode, as shown in FIGS. 1 and 2, a protection device 22, such as a diode or a neon lamp, is connected to ring and tip lines 18, 20 via relays (not shown). When a high voltage, such as an AC power signal, crosses wires 14 (FIG. 1), the current is passed through the protection device 22, thereby preventing damage to the telephone plant 12 or telephone 10.
As shown in FIG. 3, during the ringing mode, the telephone 10 is connected to ground GND via the tip line 18 and relay S1A, and the ring line 20 is connected to a ringer voltage RV by relays S1A, S1B in order to perform the telephone ringing operation. In this situation, the protection device 22 is disconnected. Thus, although protection is provided in the talking and standby modes, there is no protection provided if a power-cross condition occurs during the ringing mode.
Also, when the telephone 10 goes from an on-hook state to an off-hook state while it is ringing (i.e., when the telephone user picks up the handset), there is a need to quickly switch the relays S1A, S1B connecting the ring and tip lines 18, 20 away from the ringing bus, to the protector 22 and SLIC (SLIC not shown in FIG. 3). This switching needs to be done in order that any resistive elements on the ring and tip lines 18, 20 not become damaged due to the high ringing voltage.